What is NTP and Why is it used

Network Time Protocol (NTP) was originally developed by David L. Mills of the University of Delaware and is used to synchronize the clocks of the computer system.

NTP uses UDP port number 123, so appropriate firewall configuration or ACL is necessary for it to work properly

Suppose a network has 100+ devices. Rather than setting clock manually on each of them, NTP server can be used to control all those devices in the network, providing centralized clock management for the network.

Having wrong time in Cisco devices may lead to number of problems. For example wrong time in syslog, time based ACL, etc

Configuration Example

Because configuration is done in lab environment, the output may be different.

Set NTP server. Make sure to enter proper NTP server IP address.
You can also enter more than one NTP server,

R1(config)#ntp server ?
  Hostname or A.B.C.D  IP address of peer
  vrf                  VPN Routing/Forwarding Information

R1(config)#ntp server 192.168.1.10
R1(config)#ntp server 192.168.1.11

NTP uses Coordinated Universal Time for all time synchronizations. The router must have timezone set to display proper time

R1(config)#clock timezone JST 9

Verifying NTP Status

Show NTP status

R1#show ntp status

Verify NTP associations

R1#show ntp associations

      address         ref clock     st  when  poll reach  delay  offset    disp
 ~192.168.1.11     0.0.0.0          16     -    64    0     0.0    0.00  16000.
 ~192.168.1.10     0.0.0.0          16     -    64    0     0.0    0.00  16000.
 * master (synced), # master (unsynced), + selected, - candidate, ~ configured
R1#

Recommended Reading

Cisco documentation on NTP
Wikipedia NTP